6 BULLETIN 1402, XT. S. DEPARTMENT OF AGEICULTUEE 
the principal species, with incense cedar as a secondary species. 
Most of the middle forest belt of the west slope of the Sierras is made 
up of this group of types. 
(3) Yellow pine-white fir. — The characteristic mixed type of the 
east slope, in which sugar pine and Douglas fir are of very slight 
importance. 
(4) Sugar pine-fir, consisting of sugar pine at least 20 per cent, 
and white fir or Douglas fir the rest of the forest. It occurs on the 
best lands at the upper edge of the mixed- forest belt on the west 
slope. 
(5) White fir, white fir-red fir, and red fir. — This group of types, 
the composition of which is indicated, occupies most of the com- 
mercial timber belt above the mixed conifer belt. 
(6) Douglas fir. — This type occurs within the general range of 
the mixed conifer belt in the northern portion of the region. 
Other types are not now merchantable and are not discussed, such 
as lodgepole pine, subalpine, juniper, and digger pine-oak. 
There is a wide range of climatic and soil conditions within the 
region, and timber types vary decidedly at times within a very short 
distance, so that it is difficult or even impossible to apply one method 
of treatment to all types. 
In such t^^pes as pure yellow pine, on private lands, clear cutting 
is common ; while in the mixed types, with a high percentage of the 
firs, many trees of merchantable size are left. At the same time 
the amount of advance reproduction^ is generally greater in the 
mixed types than in the yellow pine type. Thus the tendency toward 
clear cutting is most pronounced where seed trees are most needed. 
Certain factors determining forest practice, notably the effect of 
slash fires, are common to all types; others, particularly abundance 
and distribution of potential seed trees, vary decidedly from type to 
type. In discussing methods of timber growing, types will be 
treated separately when such distinction is essential to a clear under- 
standing of the situation. 
THE ROLE OF ADVANCE REPRODUCTION AND SEED TREES 
In the virgin forest, particularly in pure yellow pine, when a good 
crop of seed is produced (and this only occurs at intervals of from 
five to eight years), the chances are slight that an equally good crop 
of seedlings will result. The long dry season and the severe frosts 
typical of the region will destroy the vast majority of the young 
seedlings during the first years of their lives. Only once in every 
10 to 25 years, on the average, does a satisfactory stand of seedlings 
become established. 
On cut-over lands, with fewer seed trees than in the virgin forest, 
new young growth will necessarily come in slowly, and it must be 
anticipated that stands of seedlings from the seed trees left after 
logging will be scattered. Seed of yellow pine is not stored in the 
duff, and reproduction can not be obtained from that source after 
logging, as it is in the Douglas fir and western white pine regions. 
1 Advance reproduction refers to the young trees growing under the old timber in 
virgin forests. Since systematic fire protection has been given our forests, this young 
growth has come in abundantly, It utilizes the space freed by the death of many old 
trees destr'oyed hy lire, insects, or disease. 
